I've just watched this footage of SLS's Mobile Launcher Platform being rolled out to LC-39B for fitting tests.

On the front of the drive trucks are two rotating discs with red markings which are clearly visible around 00:06 in the video that I haven't noticed before. I presume they are some component of the drive mechanism, but I can't work out what.

What are they and what is their function?

They are very reminiscent of disc brakes; the blue objects look a lot like brake calipers. However, if this is the case, I'm unsure why they're totally exposed rather than within the structure. For cooling?

enter image description here

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    $\begingroup$ They're disc brakes, but I haven't found a good source that says so. They're on the motor shafts (the motors are inside the gray boxes behind the discs). $\endgroup$ – Hobbes Sep 8 '18 at 18:11
  • $\begingroup$ You are right and I can't find a source either. Plenty point out the motors but not the brakes. $\endgroup$ – Organic Marble Sep 8 '18 at 20:47
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    $\begingroup$ disk brakes on cars are exposed too, other than superficial rust that poses no problems. $\endgroup$ – Hobbes Sep 9 '18 at 10:27
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    $\begingroup$ How can these relatively small brakes add significantly to the friction already present in the whole crawler assembly? $\endgroup$ – DJohnM Sep 9 '18 at 22:09
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    $\begingroup$ The brakes, along with the motors, are connected to the tracks via a geartrain that divides the motor speed by a large factor (say, 100x) and simultaneously increases motor and brake torque by that same factor. So there's a lot of stopping power in those disks. More importantly, they enable a position accuracy of 3 mm. $\endgroup$ – Hobbes Sep 10 '18 at 17:55

They are disk brakes. The two sets of pads are for service and park brakes:

The Discs attached to the shaft ends of the DC Propel motors are brake discs.


There are two different brakes on each disc. One set is parking and the other is service. Parking brakes are run by hydraulic DOT5 brake fluid. The service brakes are air brakes similar to other air brake applications on heavy duty tracks and busses. The air brakes are controlled by a pedal in the cab. The hydraulic parking brakes are controlled by a switch and it runs a electric over air system. The air is used to depress a diaphram in a cylinder that is attached to a master cylinder, similar to what is in your car. The assembly is called a "Power Cluster" and is simply air over hydraulic.

The parking brakes are smaller than the service brakes and the pads get very little wear since they apply when the crawler is stopped. The service brakes don't get used very much either, the crawler normally coasts to a stop unless there is an emergency.

Here's a CAD image of the drive train. The two shafts at the top coming forward go into the drive motors. The brakes are on the forward end of the motors.

enter image description here

From maintenance images on the NasaSpaceflight forum:

Last photo of Propel Motors with Brake Discs removed for modifications.

enter image description here

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    $\begingroup$ Although I agree with your conclusion, I don't see anything in those references that specifically say those are brakes. $\endgroup$ – Organic Marble Sep 10 '18 at 12:38
  • $\begingroup$ Yes. But they do say the crawler uses disk brakes and there are several drivetrain schematics that don't show brake disks anywhere else. $\endgroup$ – Hobbes Sep 10 '18 at 13:09
  • $\begingroup$ I was fairly sure they were brakes, but I just still can't find any schematics that actually label them! I quite like the fact that much of the information online comes from the scale model community! You can always trust the hobbyists! $\endgroup$ – Jack Sep 11 '18 at 9:35
  • $\begingroup$ Ok after a lot of searching I found them (pretty much) identified as the brake pads on nasaspaceflight here and here. You could add the maintenance photo in to your answer for reference if you want $\endgroup$ – Jack Sep 11 '18 at 11:48

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